1. Field of the Invention
This invention relates to a system and a method for monitoring blood pressure using a fabric having predetermined optical properties that respond to the motion(s) generated by geometric changes in a body due to the conduction of blood through an artery located beneath the fabric.
2. Description of the Prior Art
The “pulse” of the heart is associated with pressure pulses known to exist in the arteries carrying blood throughout the body. The periodic pumping of the heart produces these pressure pulses, which in turn flex the artery walls in rhythm with the pumping of the heart. The maximum, or peak, pressure exerted against the arterial wall occurs during the systole phase of the beat and is termed “systolic pressure”. The lowest, or baseline, pressure (known as the “diastolic pressure”) occurs during the diastole phase of the beat.
As the heart beats the pressure in the arteries fluctuates (higher during the systole phase and lower during the diastole phase of each beat), and is best described by the values for the systolic and diastolic pressures. Typical practice is to express blood pressure as a ratio of the maximum and minimum values.
The generally known method to determine these two blood pressure extremes is the auscultatory method. In this method a pressure cuff is applied to a person's upper arm. This cuff includes a bladder capable of holding air at a predetermined known pressure. The cuff bladder is inflated to a pressure above the highest expected pressure to be measured, i.e., above the systolic pressure. When inflated at this highest pressure, the cuff prevents the flow of blood in the brachial artery of the arm underlying the pressure cuff. The bladder is equipped with a valve, which allows the pressure to be reduced in a controlled way.
As air is released from the bladder, blood flow in the brachial artery is re-established. The inflow of blood through the artery is accompanied by pulsing sounds known as the Korotkoff sounds. These sounds are detected using a stethoscope at a point on the brachial artery just below the pressure cuff. The falling pressure in the cuff bladder is observed while air is released.
The Korotkoff sounds are divided in five phases based on loudness and certain qualitative features. The five phases of the Korotkoff sounds are also identified with certain pressure regimes, as normal arterial blood flow is being re-established. The first phase of the Korotkoff sounds (Phase 1) is heard at about 120 millimeters of mercury (mm Hg) characterized by a sharp “thud”; this is the systolic (maximum) blood pressure. Phase 2 is identified with a pressure of about 110 mm Hg and is heard as a swishing or blowing sound. Phase 3 is identified with a pressure of about 100 mm Hg and is described as a thud that is softer than that of Phase 1. At a pressure of about 90 mm Hg the first diastolic pressure is detected, called the Phase 4 Korotkoff sound, and heard as a softer blowing sound which disappears. Phase 5 is identified with about 80 mm Hg, and is called the second diastolic pressure. This last phase is silent, meaning that a laminar blood flow has been again established. Phase 5 may be absent in some human subjects. For this reason, the first diastolic pressure of Phase 4 is recorded as the lowest pressure in the artery.
An automated auscultatory apparatus relies on detecting sound levels and complex processing of these sounds into electronic signals, which are correlated with the phases of the Korotkoff sounds. Representative of an automated arrangement that uses an auscultatory method is the measurement system disclosed in U.S. Pat. No. 6,511,435 (Bluth et al.)
The re-establishment of blood flow in an occluded artery is also accompanied by a relatively significant flexure of the arterial wall. The flexure diminishes as the artery widens with the decrease in cuff pressure. In an alternative form of blood pressure measuring apparatus, known as an oscillatory measurement system, the mechanical vibrations accompanying arterial wall flexure are transformed into sound as they enter the inflated bladder of the cuff. This sound is detectable using a microphone located in the bladder. U.S. Pat. No. 6,458,085 (Wu et al.) discloses an oscillatory blood pressure measurement arrangement. An oscillatory blood pressure measurement arrangement is also disclosed in the above-mentioned U.S. Pat. No. 6,511,435 (Bluth et al.).
One drawback of oscillatory measuring apparatus is the reliance upon a cuff bladder modified to contain a microphone and associated connections to an external signal processor. Using a microphone to detect changes in sound pressure level and recognizing a pattern from the waveform so generated is difficult.